1. Field of the Invention
The present invention relates to an image signal processing apparatus for emphasizing a specific frequency component of an image signal and correcting the contour of a reproduced image.
2. Description of the Prior Art
Contour correction processing for emphasizing the contour of an image is one method of image quality adjustment. FIG. 1 show typical timing charts illustrating the principle of contour correction processing. FIG. 1(a) represents a luminance signal, which is-the original image signal. FIG. 1(b) is an aperture signal corresponding to a second-order differential waveform of the original image signal, where, after the second-order differential is taken of the original image signal, the signal with its polarity inverted is shown. The aperture signal fluctuates significantly at the rise and fall of the luminance signal, namely, at the edges in the image. FIG. 1(c) is the image signal after contour correction processing and is generated by adding the original image signal of FIG. 1(a) and the aperture signal of FIG. 1(b). At the image signal after contour correction processing, contour enhancement is performed so that during the rise the signal first falls then rises, and returns after exceeding a predetermined level. This emphasizes the contours of the image to improve image clarity.
FIG. 2 is a general block diagram of an aperture signal generator for generating the aperture signal. The signal that is input has a frequency component of a specific frequency band (near 1.5 MHz, for example) extracted by a bandpass filter 2. In this extraction process, noise pulses are easily created. To remove this noise, a coring circuit 4 is provided. The coring circuit 4 passes only pulses having an amplitude that exceeds a predetermined threshold and removes pulses having a lower amplitude as noise. Pulses passing the coring circuit 4 are multiplied by a predetermined gain at a gain circuit 6. Here, a second-order differential waveform generates an amplitude corresponding to the steepness of the rise and fall of the luminance signal. Namely, if the edge of the original picture is sharp, the degree of contour emphasis is increased by that much. However, excessive contour emphasis creates an unnatural image. To prevent this, a clipping circuit 8 is provided. When the amplitude of the second-order differential waveform that was gain-adjusted by the gain circuit 6 exceeds a set lower limit or upper limit, the clipping circuit 8 clips the waveform at the lower limit or upper limit.
In addition to performing the above-mentioned contour correction with respect to the luminance signal, a non-linear conversion processing is performed according to a predetermined conversion table gamma correction, the low luminance portions are enhanced, and the high luminance portions are suppressed to perform what is called gamma correction. A conventional method is described hereinafter for generating the luminance signal on which both contour correction and gamma correction are performed.
FIG. 3 is a simple block diagram of a signal processing circuit, which is a first conventional method for generating the luminance signal. The picture signal that is input, such as from an image pickup apparatus, has a frequency multiplexed luminance signal and chrominance signal, and a Y-LPF 20 is a low-pass filter that extracts the luminance signal component from the picture signal. The picture signal is input in parallel by both the Y-LPF 20 and an aperture signal generator 22. Then, the luminance signal at the output of the Y-LPF 20 and the aperture signal generated by the aperture signal generator 22 are added at an adder 24. The output signal of the adder 24 undergoes non-linear conversion at a gamma correction circuit 26 and a luminance signal is generated and output after contour correction and gamma correction.
FIG. 4 is a simple block diagram of a signal processing circuit, which is a second conventional method for generating the luminance signal. In this method, the aperture signal generator 22 generates the aperture signal on the basis of the luminance signal that is extracted at the Y-LPF 20, and is added with the output of the Y-LPF 20 at the adder 24. The output signal of the adder 24 then undergoes non-linear conversion at the gamma correction circuit 26 and a luminance signal is generated and output after contour correction and gamma correction.
FIG. 5 is a simple block diagram of a signal processing circuit, which is a third conventional method for generating the luminance signal. The picture signal is input in parallel by the Y-LPF 20 and the aperture signal generator 22. The luminance signal that is extracted at the Y-LPF 20 is input by the gamma correction circuit 26. The gamma-corrected luminance signal and the aperture signal generated at the aperture signal generator 22 are added at the adder 24. The resultant added signal is output as a luminance signal after aperture compensation and gamma correction.
FIG. 6 is a simple block diagram of a signal processing circuit, which is a fourth conventional method for generating the luminance signal. The picture signal has the luminance signal extracted at the Y-LPF 20 and the luminance signal is input by the gamma correction circuit 26. The aperture signal generator 22 generates the aperture signal on the basis of the gamma-corrected luminance signal, and this is added with the gamma-corrected luminance signal at the adder 24. The resultant added signal is output as the luminance signal after contour correction and gamma correction.
FIG. 7 shows typical signal waveforms illustrating a problem in the above-mentioned first and second methods. In the first and second methods, the aperture signal, after being combined with the luminance signal, undergoes gamma correction. FIG. 7(a) is the input signal to the gamma correction circuit 26 and shows the signal waveform where the aperture signal is combined with the luminance signal. In this input signal, the undershoot and overshoot resulting from the aperture signal both have the same magnitude of δ0. Meanwhile, FIG. 7(b) is the output signal from the gamma correction signal 26. In gamma correction, the level fluctuations in the output signal are suppressed with higher input signal levels. As a result, the magnitude δU of the overshoot after gamma correction is smaller than the magnitude δD of the undershoot. Namely, in the first and second methods, the relationship of δU<δD results so that the effects of contour correction at the high luminance side and at the low luminance side of the signal are asymmetrical. This causes a problem in which contour emphasis is relatively small at the high luminance side and relatively large at the low luminance side.
Next, FIG. 8 shows typical signal waveforms illustrating a problem in the above-mentioned third method. In the third method, after gamma correction is performed on the luminance signal, the aperture signal is combined. FIG. 8(a) shows the input signal to the gamma correction circuit 26 and the waveform in which the signal level rises in two identical P steps. The two step rise of the luminance signal is similar, and either rise has an undershoot and overshoot of the same magnitude δ as the aperture signal. Meanwhile, FIG. 8(b) shows the output signal from the adder 24 and a signal where the aperture signal is combined with the gamma-corrected luminance signal. In the level change of the two-step luminance signal, the second step level change P2′ becomes smaller than the first step level change P1′ as a result of gamma correction. Meanwhile, the aperture signal is not affected by gamma correction, and contour correction of the same magnitude δ is performed at the rise of the luminance signal at both the first step and the second step. Namely, the magnitude of contour correction is the same despite the fact that the level change of the luminance signal is P2′<P1′. This signifies that the aperture compensation is relatively large at the high luminance side and relatively small at the low luminance side, which caused a problem of visually unnatural images.
In the above-mentioned fourth method, the aperture signal is generated on the basis of the gamma-corrected image signal. Thus, the level of noise pulses created by the differentiation process of the bandpass filter 2 in the aperture signal generator 22 changes in accordance with the luminance signal level. More specifically, the noise level at the high luminance side is relatively low and the noise level at the low luminance side is relatively large. As a result, this caused a problem where the noise could not be properly removed with the coring circuit 4 having a fixed threshold.